Previous work has shown that two B apolipoproteins circulate in higher mammals, apoB-100 and apoB-48, with the latter isoprotein generated by an RNA editing process which alters the genomically encoded CAA (gln) at nucleotide 6666 to UAA. This mechanism effectively truncates the nascent protein at exactly apoB-48 size. Two projects in this laboratory have focused on apoB RNA editing. First, we have studied apoB RNA editing in the developing rat. The emergence of RNA editing occurred at 21 days in rat intestine, and 14 days in rat liver, both parallelling isoprotein data. These studies underscore the dependence of apoB-48 isoprotein secretion on the emergence of apoB RNA editing in utero. Second, we have cloned apoB cDNAs spanning 7.5 kb from the liver of an avian, Gallus domesticus, known to secrete only the apoB-100 isoprotein. Overall, humans and avians have 60% homology in the apoB cDNAs over the regions cloned. In particular, 70% homology exists between the chicken and the 23 nucleotide sequence conserved in 4 other species. In addition, the CAA (gln) codon was present at the predicted location where RNA editing may occur. Finally, we have characterized the molecular defect in a kindred from Padova, Italy, with the syndrome of familial hypobetalipoproteinemia. A Padova proband was identified with very low levels of LDL cholesterol and three discrete apoB species. An abnormal, truncated apoB protein, termed apoB-87(Padova), and normally migrating apoB-48 and apoB-100 were all detected in lipoprotein samples from the proband. A single nucleotide G deletion at the immediate 5' end of exon 28 of the apoB gene was found in the homozygous proband, resulting in a shifted reading frame which prematurely terminates translation after amino acid 3978, providing the mechanism for apoB-87 synthesis. The mechanism for production of an apparently normally sized apoB-100 in the homozygous apoB-87(Padova) proband is unknown. This is the first report of a homozygous apoB gene defect in familial hypobeta-lipoproteinemia with detectable circulating apoB.